A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a grid of adjacent target portions referred to as “fields” that are successively patterned. Known lithographic apparatus include so-called steppers, in which each field is irradiated by exposing an entire field pattern onto the field at one time, and so-called scanners, in which each field is irradiated by scanning the field pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The pattern is imaged onto the target portion of the substrate using lenses (or mirrors) forming a projection system. When imaging the pattern onto the substrate it is desirable to ensure that an uppermost surface of the substrate (i.e. the surface onto which the pattern is to be imaged) lies within the focal plane of the projection system.
The surface of a substrate on which a pattern should be projected is never perfectly flat, but presents many height deviations on both a large scale and smaller scale. Failure to adjust the focus of the projection system may result in poor patterning performance and consequently poor performance of the manufacturing process as a whole. Performance parameters such as critical dimension (CD) and CD uniformity in particular will be degraded by poor focus.
To measure these height deviations, height sensors are normally integrated in the lithographic apparatus. These are typically optical sensors used to measure and the vertical position of the uppermost surface of the substrate at points all across the substrate, after it has been loaded into the lithographic apparatus. This set of measurements is stored in some suitable form and may be referred to as a “height map”. The height map is then used when controlling imaging of the pattern onto the substrate, to ensure that the radiation sensitive resist layer on each portion of the substrate lies in the focal plane of the projection lens. Typically the height of a substrate support bearing the substrate will be adjusted continuously during exposure of successive portions on a substrate. Examples of height sensors are disclosed in U.S. Pat. No. 7,265,364, U.S. patent application publication no. 2010/0233600 and U.S. patent application publication no. 2013/0128247. They do not need to be described in detail herein. Corrections to the height map may be made using other sensor measurements (for example an air gauge) to reduce process dependency of the measurements. According to PCT patent application publication no. WO2015131969 (which claims priority to European patent application EP14157664.5, which was not published at the earliest priority date), further corrections can be applied to the height map based on prior knowledge of product design and process dependency.